Individual tumors exhibit distinct characteristic natures, and their biological properties are not necessarily identical even though the basic principle of oncogenesis is the same. Rapid advances in the understanding of cancer from a molecular biological and molecular genetic perspective in recent years have opened the way to an explanation of oncogenesis and tumor cell biology on the genetic level.
(Neuroblastomas)
Neuroblastoma is a pediatric cancer occurring in sympathetic gangliocytes and adrenal medullary cells which originate from cells of the peripheral sympathetic nervous system. Of these sympathetic nervous system cells, neural crest cells in the initial stage of development migrate to the abdomen, differentiating and maturing at sites where sympathetic ganglia are formed. Some of these cells migrate further to the adrenal bodies, penetrating through the adrenal cortex which is already in the process of formation, and reaching the medulla and forming medullary substance there. The neural crest cells also serve as a source of other peripheral nerve cells, differentiating into dorsal root ganglia (sensory nerves), skin pigment cells, thyroid C cells, some pulmonary cells, intestinal gangliocytes, and the like.
(Prognosis for Neuroblastoma)
Neuroblastoma is characterized by a varied clinical profile (Nakagawara, Shinkeigashu no Hassei to Sono Bunshi Kiko [Neuroblastoma Development and Molecular Mechanism], Shoni Naika 30, 143, 1998). For example, neuroblastomas occurring at less than one year of age have very favorable prognosis, with the majority undergoing differentiation and cell death, and spontaneous regression. Currently, most neuroblastomas discovered by a positive result in the commonly performed mass screening of 6-month-old infant urine are of the type which tend to undergo this spontaneous regression. On the other hand, neuroblastomas occurring at age 1 or higher are highly malignant and lead to death of the infant in the majority of cases. It is also hypothesized that a somatic mutation occurs in highly malignant neuroblastomas in infants older than one year of age, which are of monoclonal nature, whereas in naturally regressing neuroblastomas, the genetic mutation remains at only a germline mutation. See Knudson A G, et al.: Regression of neuroblastoma IV-S: A genetic hypothesis, N. Engl. J. Med. 302, 1254 (1980)).
(Tumor Markers which Allow the Diagnosis of Prognosis for Neuroblastoma)
With recent advances in molecular biology research, it has become clear that expression of the high affinity nerve growth factor (NGF) receptor TrkA is closely connected with control of differentiation and cell death. See Nakagawara A., The NGF story and neuroblastoma, Med. Pediatr. Oncol., 31, 113 (1998). Trk is a membrane-spanning receptor, existing as the three main types, Trk-A, -B and -C. These Trk family receptors play an important role in specific nerve cell differentiation and survival in the central nervous and peripheral nervous systems. See Nakagawara, et al., Shinkeigasaiboushu ni Okeru Neurotrophin Juyoutai no Hatsugen to Yogo [Expression of Neurotrophin Receptors and Prognosis in Neuroblastoma], Shoni Geka (Pediatric Surgery), 29:425–432, 1997. The survival and differentiation of tumor cells is controlled by signals from Trk tyrosine kinase and Ret tyrosine kinase. In particular, the role of TrkA receptor is most significant, with TrkA expression being notably high in neuroblastomas with favorable prognosis, and its signals exerting a powerful control over survival and differentiation of tumor cells, and cell death (apoptosis). In neuroblastomas with unfavorable prognosis, on the other hand, TrkA expression is significantly suppressed, while tumor development is aided by a mechanism in which survival is promoted by signals from TrkB and Ret.
It has become clear that amplification of the neural oncogene N-myc has become clearly associated with the prognosis of neuroblastoma. See Nakagawara, Nou-shinkeishuyo no Tadankai Hatsugan [Multistage Oncogenesis of Cerebral and Neural Tumors], Molecular Medicine, 364, 366(1999). This gene, first cloned in neuroblastoma, is ordinarily only present in a single copy per haploid set in normal cells and neuroblastomas with favorable prognosis, whereas it has been found to be amplified several dozen times in neuroblastomas with unfavorable prognosis. Thus, amplification of N-myc is closely linked to tumor progression.
Up till the present time, however, no oncogene other than N-myc is known to be expressed in neuroblastomas, and absolutely no genetic information other than that of N-myc has been known in relation to favorable or unfavorable prognosis.